JP3156525B2 - Manufacturing method of steel plate for welded structure with excellent cross weld joint characteristics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a steel plate for use in a portion where a tensile stress in the thickness direction due to welding or a stress due to diffusible hydrogen acts, such as a through plate of a cruciform welded joint of a steel structure. The present invention relates to a method for manufacturing a 60-kg class steel sheet in which defects such as cracks do not occur in a thick central portion.

[0002]

2. Description of the Related Art Steel used for steel structures is inexpensive because it is used in large quantities, and at the same time, it is necessary to prevent low-temperature cracking during small heat input welding such as tack welding and to reduce preheating work during welding. It is required to have a possible so-called good weldability. In order to satisfy these demands, controlled rolling and accelerated cooling capable of securing strength and toughness with a small alloy content have been developed, and various technologies based on combinations of rolling conditions and cooling conditions have been put to practical use.

[0003] One of the above techniques, a two-phase area rolling method,
That is, a method of lowering the rolling temperature to the Ar ₃ point or lower in controlled rolling to improve the strength (for example, Japanese Patent Application Laid-Open No.
No. 58019) is a method of manufacturing a steel material in which strength is more important than toughness, and a 60-kilometer steel plate or the like is manufactured by this method.

[0004]

Cross welded joints are often used in steel structures. FIG. 1 is a schematic longitudinal sectional view for explaining a crack generated in a through plate of a cruciform welded joint in an actual structure. In the figure, 1 is a penetration plate, 2 is a crack, 3
Is a restraint plate, and 4 is a weld metal (multi-layer welding). As shown in FIG. 1, it is conventionally known that, in an actual structure, a large crack 2 parallel to the plate surface may occur at the center of the plate thickness of the through plate 1. However, recently, as a countermeasure for preventing such parallel cracks from being generated on a sheet surface in a two-phase region rolled steel material having a strength of 60 kg, an S content of 0.002 wt. It is considered that a Z35 grade steel material of WES3008 having an aperture value of more than 70% in a thickness direction tensile test (hereinafter, referred to as a Z direction tensile test) is effective. However, even in the case of the Z35 grade steel material of WES3008, it has become clear that the above-mentioned cracks have occurred in the through plate of the cruciform welded joint, which is a major problem. On the other hand, recently, with the increase in the size of steel structures, the use of 60 kg-class steel has been increasing.

[0005] Accordingly, an object of the present invention is to solve the above-mentioned problems and to use the present invention for a penetration plate of a cruciform welded joint of a steel structure.
It is an object of the present invention to provide a method for manufacturing a steel plate for a welded structure of a 60-kilometer class in which a crack is not generated at a central portion of the plate thickness. In order to develop a 60-kg class steel that can be used, various manufacturing conditions were studied mainly for the two-phase region material.

[0006]

Means for Solving the Problems First, the inventors of the present invention used a small window type test which has been conventionally employed in a cruciform welded joint of an actual structure from a base steel plate having cracks as shown in FIG. A piece was taken and tested, but no cracks occurred.
The base material is a WES3008Z35 grade two-phase region rolled steel sheet, and a small window type test piece is, for example, “Journal of the Japan Welding Society” Vol. 5, No. 3, p. 239. It is considered that since the test piece was smaller than the actual structure, no large stress was applied to the center of the sheet thickness where the component segregation zone due to continuous casting was present, and thus no crack was generated.

Therefore, first, in order to reproduce a crack at the center of the plate thickness similar to a crack in an actual structure, various test methods were tried, and the following test method was devised. FIGS. 2, 3 and 4 are a plan view, a front view and a side view of a window-type specimen with a notch employed in the test of the present invention, and FIG.
It is a detailed view of -A section. In these figures, reference numeral 1a denotes an insertion test plate, which is a steel plate corresponding to a through plate in an actual structure. 3 is a restraining plate, 4 is a weld metal (multi-layer welding), and 5 is a notch, and the shape is a 2 mm V notch. As shown in FIG. 5, a notch 5 is provided at the center of the thickness of one end face of the insertion test plate 1a, and the insertion test plate 1a is inserted into the restraint plate 3 so that the end face provided with the notch 5 does not protrude. Both plates were welded.

[0008] The welding conditions are as follows. Welding method: CO ₂ multi-layer welding, welding material: 60 kg class flux cored wire, 1.
6mmφ, Condition: First layer 150A-26V-18cm / mi
n, 2nd layer 200A-29V-18cm / min, without preheating, temperature between passes 100 ° C, welding pass schedule: 38 passes in total, welding order: as shown in FIG. 2
After welding in the order of → 3 → ─── → 8, weld in the same order in the order of →→.

When a test was carried out under the above welding conditions using the notched window type test specimen described with reference to FIGS. 2 to 5, cracks similar to those generated in the cruciform welded joint of the actual structure were found on the insertion test plate. It was confirmed that it would occur. Hereinafter, this method is referred to as a notched window cross test.

FIG. 7 is a schematic vertical cross-sectional view of a welded test piece showing a state of a crack generated by the notched window type cross test method, and corresponds to the AA cross-sectional view in FIG. . The crack 2 is generated from the vicinity of the notch bottom and propagates brittlely in the central portion of the plate thickness. Observation of the crack origin with a scanning electron microscope (hereinafter, referred to as SEM) reveals that MnS inclusions are present there, and the vicinity of the periphery shows a hydrogen-like pseudo-cleavage surface, and cracks in the actual structure Was reproduced.

From the above test results, it can be seen that the notched window-type cruciform test method successfully reproduces cracks in a cruciform welded joint of an actual structure.

[0012] Then, as a result of evaluating a steel plate manufactured by various manufacturing methods by a window type cross test with a notch, it was found that the following three measures had to be taken to prevent the occurrence of cracks. In the process of continuous casting, light reduction is performed to reduce central segregation, Ca is added, sulfide-based inclusions are made spherical, and rolling conditions are controlled to obtain a (001) <110> texture. To reduce The method for producing a 60-kg class steel sheet capable of preventing the occurrence of cracks in the cross weld is specifically described below.

The first invention of the method for producing a steel plate for a welded structure having excellent cross weld joint characteristics according to the present invention is characterized in that carbon (C): 0.06 to 0.09 wt. %, Silicon (Si): 0.5 wt. % Or less, manganese (Mn): 1.0 to 1.7 wt. %, Phosphorus (P): 0.02 wt. % Or less, sulfur (S): 0.002 wt. % Or less, calcium (Ca): 0.5 to 2.0 as Ca / S, boron (B): 0.0002 wt. % Or less, and soluble aluminum (sol. Al): 0.005 to 0.060 wt. %, And niobium (Nb): 0.005 to 0.04 wt. %, Vanadium (V): 0.02 to 0.1 wt. %, And titanium (Ti): 0.005 to 0.10 wt. %, And at least one element selected from the group consisting of precipitation hardening elements, and the following formula (1): PCM = C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1) The PCM represented is 0.20 wt. % Or less, the balance being substantially low iron and low alloy steel of low alloy continuously consisting of iron (Fe) under light pressure is prepared.
It is heated to a temperature of 050 ° C. or higher and hot rolled. In the hot rolling, the cumulative rolling reduction is 3 at a temperature lower than the Ar3 transformation point.
It is characterized in that a reduction in the range of 0 to 70% is performed and the hot rolling is completed at a temperature of 650 ° C. or more. In a second aspect, the low-carbon low-alloy steel is copper (Cu): 0.5 wt. % Or less, nickel (Ni): 0.5 wt. % Or less, chromium (Cr): 0.5 wt. % Or less, and molybdenum (Mo): 0.3 wt. % Or less, characterized by further containing at least one of the following.

The third invention of the method for producing a steel plate for a welded structure having excellent cross weld joint characteristics according to the present invention is characterized in that carbon (C): 0.06 to 0.09 wt. %, Silicon (Si): 0.5 wt. % Or less, manganese (Mn): 1.0 to 1.7 wt. %, Phosphorus (P): 0.02 wt. % Or less, sulfur (S): 0.002 wt. % Or less, calcium (Ca): 0.5 to 2.0 as Ca / S, boron (B): 0.0002 wt. % Or less, and soluble aluminum (sol. Al): 0.005 to 0.060 wt. %, And niobium (Nb): 0.005 to 0.04 wt. %, Vanadium (V): 0.02 to 0.1 wt. %, And titanium (Ti): 0.005 to 0.10 wt. %, And at least one element selected from the group consisting of precipitation hardening elements, and the following formula (1): PCM = C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1) The PCM represented is 0.20 wt. % Or less, the balance being substantially low iron and low alloy steel of low alloy continuously consisting of iron (Fe) under light pressure is prepared.
It is heated to a temperature of 050 ° C. or higher to perform hot rolling. In the hot rolling, the rolling is completed at a temperature of the Ar3 transformation point or higher, and immediately after the end of the rolling, water is cooled at a cooling rate of 3 ° C./S or higher. , The water cooling is stopped at a temperature within the range of 400 to 600 ° C., and then the mixture is allowed to cool. In a fourth aspect of the present invention, the low-carbon low-alloy steel comprises copper (Cu): 0.5 wt. % Or less, nickel (Ni): 0.5 wt. % Or less, chromium (Cr): 0.5 wt. % Or less, and molybdenum (Mo): 0.3 wt. % Or less, characterized by further containing at least one of the following.

[0015]

The reason for limiting the present invention as described above will be described below. The present inventors prepared a steel sheet having a thickness of 18 mm having the chemical composition of Nos. A1 to A10 and the manufacturing conditions shown in Tables 1 and 2. The chemical composition is C
Except for the presence or absence of a and the S content (there are two levels of 0.002 wt.% and 0.0007 wt.%), it is almost the same for all steel sheets. The two-phase region temperature is included in the temperature below the Ar ₃ transformation point, and the austenite (γ) region and the temperature above the Ar ₃ transformation point have the same meaning.

[0016]

[Table 1]

[0017]

[Table 2]

The table will be supplementarily described with respect to the manufacturing conditions of the steel sheet. Hereinafter, in this specification, for example, steel sheet NO.A
1 is simply denoted by A1. A1: Cumulative rolling reduction of 3 in light phase continuous casting slab in two-phase region
Rolling was performed at 0%, and the rolling was completed at 670 ° C. A2: Continuously cast slab under light pressure is austenitic (γ)
And rolled at 670 ° C. A3: Continuous casting slab under light pressure is austenitic (γ)
Rolled in the area and accelerated cooling. A4: The continuous reduction slab with light addition of Ca was subjected to rolling at a cumulative rolling reduction of 30% in the two-phase region, and the rolling was completed at 690 ° C. A5: The continuous reduction slab under light rolling with Ca added was subjected to rolling at a cumulative rolling reduction of 70% in the two-phase region, and the rolling was completed at 670 ° C. A6: The continuous reduction slab under light rolling to which Ca was added was subjected to rolling at a cumulative rolling reduction of 80% in the two-phase region, and the rolling was completed at 640 ° C. A7: A continuous cast slab to which Ca was added but not subjected to light rolling was subjected to rolling at a cumulative rolling reduction of 70% in a two-phase region,
The rolling was completed at 670 ° C. A8: The continuous casting slab under light pressure to which Ca was added was rolled in the γ region and accelerated cooled. A9: The continuous cast slab to which Ca was added but not subjected to light reduction was rolled in the γ region and accelerated cooled. A10: The continuous reduction slab under light pressure with the sulfur content extremely reduced to 0.0007 wt.% Was subjected to rolling at a cumulative rolling reduction of 70% in the two-phase region, and the rolling was completed at 670 ° C.

In the above, the steel plate with the slab subjected to slight reduction is aimed at reducing the component segregation at the center of the plate thickness, and the steel plate subjected to ordinary hot rolling (A2) From the viewpoint of the generation mechanism, the aim is to relax the (001) <110> texture and reduce the elongation of MnS inclusions.-A steel sheet (A3) that has been subjected to accelerated cooling after rolling in the γ region
Is the (001) <110> texture relaxation and MnS
A4 to A9 steel sheets to which Ca is added aim at reducing the elongation of inclusions and improve strength, and aim at spheroidization of MnS inclusions. The A10 steel sheet is intended to reduce the MnS content.

Table 3 shows the results of a normal tensile test, a Z-direction tensile test, and a notch window type cross test performed on each of the steel plates shown in Tables 1 and 2.

[0021]

[Table 3]

From the table, the following matters can be understood. Except for the normal rolled steel sheet (NO. A2), the steel sheet satisfies the required strength (TS ≧ 570 MPa) as a 60 kg steel sheet. Further, the drawing value of the tensile in the Z direction is 70% or more for all the steel sheets, which sufficiently satisfies the Z35 class of WES3008. However, the presence or absence of cracks in the notched window type cross test differs depending on the method of manufacturing the steel sheet.

In order to prevent the above cracks, Ca is added, casting is performed under light pressure, control of the two-phase region rolling conditions or γ
It is necessary to perform accelerated cooling from the area. That is, in order to prevent the occurrence of cracks, Ca is added to steel to form spheroids of MnS inclusions, light reduction is performed in continuous casting to reduce central segregation, and control is performed under appropriate rolling conditions. (001) <110> to relax the texture
It turned out that it was necessary to take all three measures.

Next, the reasons for limiting the chemical composition of the present invention will be described. (1) C: C is an element necessary for securing the strength of the steel material. If the C content is less than 0.06 wt.%, TS> 5
In order to satisfy 70 MPa, it is necessary to add a large amount of alloying elements, resulting in high cost. On the other hand, if it exceeds 0.09 wt.%, The tack weldability deteriorates. Therefore, the C content is
It was limited to the range of 0.06 to 0.09 wt.%.

(2) Si: Si is an element necessary for the strength of steel material, preliminary deoxidation of molten steel, etc.
If it exceeds 5 wt.%, The toughness of the steel material and the toughness of the welded HAZ deteriorate. Therefore, the content of Si is limited to 0.5 wt.% Or less.

(3) Mn: Mn is an element necessary for securing the strength of the base material. In order to ensure TS> 570 MPa, the content needs to be 1.0 wt.% Or more. On the other hand, since Mn is an element that is easily segregated at the center,
If it is added in excess of 1.7 wt.%, The center of the sheet thickness is remarkably hardened, which promotes the occurrence of cracks in the cross weld. Therefore, the Mn content was limited to the range of 1.0 to 1.7 wt.%.

(4) P: P is an unnecessary element in the steel material of the present invention, and the lower the content, the better. Further, it is an element that is very easily segregated at the center, and its content is 0.02.
If the content exceeds wt.%, the center of the sheet thickness is remarkably hardened, which promotes the occurrence of cracks in the cross weld. Therefore, the P content is 0.02
It must be limited to wt.% or less.

(5) S: S exists as sulfide in solid steel. When an appropriate amount of Ca is added, it is present as CaS. CaS does not elongate by rolling and exists as spherical inclusions (C-based inclusions) in the steel sheet. However, when the S content exceeds 0.002 wt.%, The spherical inclusions are arranged in a row in the rolling direction, so-called cluster (B).
System inclusions). The B-based inclusion can be a starting point of a crack in the cross weld. Therefore, the S content is 0.0
It was limited to 02 wt.% Or less.

(6) Ca: Ca is added to spheroidize elongated sulfide (MnS) inclusions, which are the starting points of cracks in the cross weld. In order to make the sulfide inclusions spherical, it is necessary to add Ca so that the Ca content is 0.5 times or more the S content in the chemical composition of the steel material.
On the other hand, in the chemical composition of the steel material, when Ca having a Ca content that is more than twice the S content is added, CaS is connected in a row, and so-called clusters (B-based inclusions) are easily formed. Therefore, when the Ca content is 0.5 to 2 Ca (wt.%) / S (wt.%) (Referred to as Ca / S in this specification).
Within the range.

(7) Al: Al is an element necessary for deoxidation. If the sol.Al content is 0.005 wt.% or less, there is no sufficient deoxidizing effect. On the other hand, if the content is more than 0.06 wt.%, The purification of steel is deteriorated, and it is combined with CaS to form Al-C
a-Sulfur oxide clusters (B-based inclusions) are formed, which can be the starting point of cracks in the cross weld. Therefore, the sol.Al content was limited to the range of 0.005 to 0.06 wt.%.

(8) B: B is an unnecessary element in the steel material. Since even a small amount of B significantly deteriorates tack weldability, the B content is limited to 0.0002 wt.% Or less.

(9) Nb, V and Ti: Nb, V and Ti are elements which can precipitate carbonitrides by adding a small amount and increase the strength, and are steels having a low alloying element content. It is an element necessary for securing a strength of 60 kg class. However, if the content is too small, the effect is not clear, while if the content is excessive, the HAZ toughness is significantly deteriorated. Therefore, from the above viewpoint, the Nb content is 0.005 to 0.04 wt.%, The V content is 0.02 to 0.1 wt.%, And the Ti content is
It was limited to each range of 0.005 to 0.1 wt.%.

(10) Cu, Ni, Cr and Mo: C
u, Ni, Cr and Mo are elements that can increase the strength by increasing the hardenability, and are useful elements for securing a strength of 60 kg class. However, when these elements are added excessively, the hardness of the central segregation part is remarkably increased, and the generation of cracks in the cross weld part is promoted. In view of the above, the contents of Cu, Ni and Cr are all 0.5 wt.% Or less (including the case without addition), and the Mo content is 0.3 wt.% Or less (including the case without addition). Limited to.

[0034] (11) P _CM: P _CM value expressed by the following equation (1) is an indicator of tack weldability. When _{P CM = C + Mn / 20} + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ──── (1) P CM value exceeds 0.20 wt.%, When welded without preheating during tack welding, There is a risk that root cracks occur. Therefore, P _CM value was limited to 0.20 wt.%. Incidentally, P _CM value is preferably not more than 0.18 wt.%.

The reason why the steel slab for hot rolling the steel sheet is limited to the continuous cast slab is that it is advantageous from the viewpoint of productivity and cost. Furthermore, it is one of the features of the present invention to apply light reduction in the continuous casting process. By subjecting the slab to several percent reduction at the end of solidification, the phenomenon of molten steel in which the alloy elements are concentrated is prevented from convectively macroscopically in the slab crater is reduced. The concentration of alloying elements such as Mn, P and S is reduced. There is a so-called center segregation reducing effect. Even when Ca is added, since casting under light rolling is not performed, if the central segregation of S is remarkable, huge CaS inclusions or cluster-like CaS inclusions are easily formed. In such a steel plate, cracks occur in the notched window cross test (see A7 and A9 in Table 3).

Next, the reasons for limiting the rolling and cooling conditions of the steel sheet will be described. In order to secure the strength of the steel sheet, it is important to sufficiently dissolve the carbon and nitride in the slab before rolling. Therefore, the rolling heating temperature was limited to 1050 ° C. or higher.

The steel according to the method of the present invention is, P _CM ≦ 0.20w
Since the content of alloying elements is low and the content of alloying elements is low, in order to secure sufficient strength as a 60 kg class steel, ferrite is work-hardened by two-phase rolling or accelerated cooling to form a bainite-based structure. Is required.

First, in the case of two-phase zone rolling, if the cumulative rolling reduction is less than 30%, the above-mentioned work hardening is insufficient, and P _CM ≦
Within the range of 0.20 wt.%, It is difficult to satisfy TS ≧ 570 MPa. On the other hand, when the cumulative rolling reduction exceeds 70%,
(001) <110> Texture remarkably develops, and cracks occur in the cruciform weld.

When the rolling finishing temperature is lower than 650 ° C., the (001) <110> texture is remarkably developed, and the cruciform weld is liable to crack.

From the above, as the conditions for the two-phase rolling, the cumulative rolling reduction was limited to a range of 30 to 70% and the hot rolling finishing temperature was limited to 650 ° C. or higher.

Next, in the case of γ-region rolling, when rolling is completed in the γ region, no (001) <110> texture is formed,
It is effective in suppressing the occurrence of cracks in the cross weld. Further, after that, the strength can be improved by accelerated cooling. The P _CM ≦ 0.20wt.% Of low alloy steels, the cooling rate 3 °
When accelerated cooling at less than C / s, the area ratio of ferrite increases, making it difficult to satisfy TS ≧ 570 MPa. However, even when accelerated cooling at a cooling rate of 3 ° C / s,
If the accelerated cooling stop temperature is higher than 600 ° C, the area ratio of ferrite is increased, and it becomes difficult to satisfy TS ≧ 570 MPa. On the other hand, if the accelerated cooling stop temperature is lower than 400 ° C, the central segregation portion of the steel sheet is very small. Hardens and promotes hydrogen cracking.

As described above, the conditions for accelerated cooling in the case of γ-region rolling are as follows: immediately after rolling in the γ-region finish, accelerated cooling is performed at a cooling rate of 3 ° C./s or more. The cooling was stopped at the temperature.

The steel sheet manufactured according to the present invention can be used not only in the penetration plate of the cruciform welded portion but also in a part having the same use condition as that of the steel plate, that is, in a welded joint part where a tensile stress acts in the thickness direction of the steel sheet. Needless to say, they also exhibit excellent effects.

[0044]

Next, a method for manufacturing a steel sheet for a welded structure according to the present invention and a steel sheet obtained by the method will be described by way of examples.
This will be described in more detail. Tables 4 and 5 show that a steel sheet manufactured under the conditions within the scope of the present invention (hereinafter referred to as the present invention test material) and a steel sheet manufactured under conditions outside the scope of the present invention (hereinafter referred to as the comparative test material). ) Shows the chemical composition, the slab casting method, the rolling and cooling conditions of the steel sheet, and the sheet thickness.

[0045]

[Table 4]

[0046]

[Table 5]

[0047] From each sample of the same table, to prepare a predetermined specimen, tensile strength (TS), Charpy - impact test (absorption energy of _{0 ° C -: V E 0} ), Z -direction tensile test (aperture Value), an oblique y-shaped welding crack test (preheating temperature for preventing cracking), and a window-shaped cross test with a notch. The oblique y-shaped weld cracking test was performed according to JISZ3158. Table 6 shows the test results. The table shows the following items.

[0048]

[Table 6]

The test material of the present invention has a sufficient strength (TS ≧ 570 MPa) as a 60 kg class steel. Good toughness. However, accelerated coolant (C1, E
1, G1, H1, K1 and L1) show slightly better toughness values than the two-phase rolled material (B1, D1, F1, I1, J1 and M1). The aperture value in the Z direction is as good as 70% or more. Preheating temperature for preventing cracking is also 25 °
C or less, and it was confirmed that welding could be performed without preheating. Regarding the prevention of cracks in the cruciform welds, which is the subject of the present invention, no cracks were generated in the notch cross-shaped cruciform test, which is very excellent.

On the other hand, for the test material for comparison,
Cracks and the like at the cruciform joint were observed as shown below, and it was inferior as a steel plate for welded structures. For B2 to M2, the chemical composition is within the scope of the present invention, but other conditions are as follows:
It is outside the scope of the present invention. -Since B2 and C2 were not subjected to light reduction in the continuous casting process, cracks occurred in the notched window-type cross test. D2 has a crack in a notched window cross test because the two-phase rolling condition is outside the range of the present invention (the cumulative rolling reduction is excessive and the rolling finish temperature is too low). -Since E2 did not perform light reduction casting, cracks occurred in the notched window cross test. Further, the strength is insufficient because the accelerated cooling rate is too slow. -G2 is subjected to accelerated cooling to 100 ° C after rolling in the two-phase region, so that the central segregated portion is extremely hardened and cracks are generated in a notched window cross test. Also, the toughness value is inferior.・ G3 has not been subjected to light pressure casting, so cracks have occurred in the notch cross-shaped window test. Further, since the cumulative rolling reduction in the two-phase region is insufficient, the strength of the 60-kg class steel is insufficient. -L2 is also accelerated and cooled to 200 ° C, the center segregation part is extremely hardened, cracks are generated in a notched window cross test, and the toughness is poor.・ No.M2 has not been subjected to light pressure casting, so cracks have occurred in the notch cross-shaped window test. In addition, since heating was performed at a low temperature of 1000 ° C., rolling was performed in a state where carbon and nitride were not sufficiently dissolved, the effect of precipitation strengthening after rolling was reduced, and the strength was insufficient.

Regarding the following N2 to Q2, rolling conditions and cooling conditions are within the scope of the present invention, but the chemical composition is outside the scope of the present invention, and the casting conditions are also outside the scope of the present invention. Things. N2 adds Ca to make sulfides spherical, but Ca / S is too large to form CaS clusters. For this reason, cracks have occurred in the notched window cross test. -Since O2 is not subjected to light reduction casting, cracks have occurred in a notch window type cross test. Furthermore, since P _CM value is too high, oblique y-break preventing preheating temperature is inferior high as 50 ° C.・ P2 has no MnS inclusions extending in the rolling direction because Ca content is not added even though the S content is very low, that is, 0.0006 wt.%. Has occurred. -In Q2, the casting under light pressure was not performed, and the C content was too high, so that the central segregated portion was hardened and cracks were generated in a notched window cross test. Also, since P _CM value is too high, oblique y-break preventing preheating temperature is inferior high as 50 ° C.

As described above, any of the steel sheets manufactured under the conditions within the scope of the present invention does not crack in the notch cross-shaped cross test, and is excellent as a 60 kg-class steel sheet for welded structures. It was. On the other hand, a steel sheet manufactured under any one of the conditions out of the range of the present invention cracked in the notch cross-shaped cross test. Some of these steels lacked strength, were poor in toughness, or were poor in y-break prevention preheating temperature as a 60-kg class steel.

[0053]

As described above, according to the present invention,
Relates to a process for the preparation of P _CM ≦ 0.20wt.% 60-kilogram steel sheet assumes a low chemical composition of the alloy component content of,
The sulfide inclusions are made spherical by limiting the Ca content to within an appropriate range, reduce the central segregation by applying light reduction in continuous casting, and secure the strength by optimizing the two-phase region rolling conditions or accelerated cooling conditions. In addition, the formation of excessive (001) <110> texture was suppressed. As a result, the penetration plate of the cross weld, T
An industrially useful effect that can provide a method for manufacturing a steel plate for a welded structure that can suppress the occurrence of cracks even when used in a U-shaped welded portion or a helicoidal welded portion is provided.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view illustrating a crack generated in a through plate of a cross weld joint in an actual structure.

FIG. 2 is a schematic plan view of a window-type specimen with a notch employed in the test of the present invention.

FIG. 3 is a schematic front view of a window-type specimen with a notch employed in the test of the present invention.

FIG. 4 is a schematic side view of a window-type specimen with a notch employed in the test of the present invention.

5 is a detailed view of an AA cross section of FIG. 2, showing a state in which an insertion test plate is inserted into a restraining plate.

FIG. 6 is a schematic longitudinal sectional view for explaining a welding pass schedule in a window type cross test with a notch employed in the test of the present invention.

FIG. 7 is a schematic vertical cross-sectional view of a welded test piece for explaining a state of a crack generated in a notched window type cross test adopted in the test of the present invention.

[Explanation of symbols]

 1 penetrating plate, 1a insertion test plate, 2 crack, 3 restraint plate, 4 weld metal, 5 notch, 6 gap.

Continuation of the front page (56) References JP-A-61-270333 (JP, A) JP-A-63-317624 (JP, A) JP-A-2-254119 (JP, A) JP-A-3-44417 (JP) , A) JP-A-5-239594 (JP, A) JP-A-5-212439 (JP, A) Zen, Ohno, Sekino, Iwasaki, "Fatigue strength of cruciform welded joints with unwelded defects", National Meeting of the Japan Welding Society Summary of lecture, No. 31, P. 272-273 (1982) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00-8/02

Claims (4)

(57) [Claims] 1. Carbon (C): 0.06 to 0.09 wt. %, Silicon (Si): 0.5 wt. % Or less, manganese (Mn): 1.0 to 1.7 wt. %, Phosphorus (P): 0.02 wt. % Or less, sulfur (S): 0.002 wt. % Or less, calcium (Ca): 0.5 to 2.0 as Ca / S, boron (B): 0.0002 wt. % Or less, and soluble aluminum (sol. Al): 0.005 to 0.060 wt. %, And niobium (Nb): 0.005 to 0.04 wt. %, Vanadium (V): 0.02 to 0.1 wt. %, And titanium (Ti): 0.005 to 0.10 wt. %, And at least one element selected from the group consisting of precipitation hardening elements, and the following formula (1): PCM = C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1) The PCM represented is 0.20 wt. % Or less, the balance being substantially low iron and low alloy steel of low alloy continuously consisting of iron (Fe) under light pressure is prepared.
It is heated to a temperature of 050 ° C. or higher and hot rolled. In the hot rolling, the cumulative rolling reduction is 3 at a temperature lower than the Ar3 transformation point.
Subjected to pressure in the range of 0% to 70%, characterized by terminating the hot rolling at 650 ° C or higher, cross soluble
Manufacturing method of steel plate for welded structure with excellent joint characteristics . 2. The low-carbon low-alloy steel comprises copper (Cu): 0.5 wt. % Or less, nickel (Ni): 0.5 wt. % Or less, chromium (Cr): 0.5 wt. % Or less, and molybdenum (Mo): 0.3 wt. % Or less, at least one of the following:
Manufacturing method of steel plate for welded structure with excellent cross weld joint characteristics . 3. Carbon (C): 0.06 to 0.09 wt. %, Silicon (Si): 0.5 wt. % Or less, manganese (Mn): 1.0 to 1.7 wt. %, Phosphorus (P): 0.02 wt. % Or less, sulfur (S): 0.002 wt. % Or less, calcium (Ca): 0.5 to 2.0 as Ca / S, boron (B): 0.0002 wt. % Or less, and soluble aluminum (sol. Al): 0.005 to 0.060 wt. %, And niobium (Nb): 0.005 to 0.04 wt. %, Vanadium (V): 0.02 to 0.1 wt. %, And titanium (Ti): 0.005 to 0.10 wt. %, And at least one element selected from the group consisting of precipitation hardening elements, and the following formula (1): PCM = C + Mn / 20 + Si / 30 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1) The PCM represented is 0.20 wt. % Or less, the balance being substantially low iron and low alloy steel of low alloy continuously consisting of iron (Fe) under light pressure is prepared.
It is heated to a temperature of 050 ° C. or higher to perform hot rolling. In the hot rolling, the rolling is completed at a temperature of the Ar3 transformation point or higher, and immediately after the end of the rolling, water is cooled at a cooling rate of 3 ° C./S or higher. stops the water cooling at a temperature in the range of 400 to 600 ° C, characterized by subsequently cooling, cross welding
Manufacturing method of steel plate for welded structure with excellent joint characteristics . 4. The low-carbon low-alloy steel comprises copper (Cu): 0.5 wt. % Or less, nickel (Ni): 0.5 wt. % Or less, chromium (Cr): 0.5 wt. % Or less, and molybdenum (Mo): 0.3 wt. % Or less, at least one of the following:
Manufacturing method of steel plate for welded structure with excellent cross weld joint characteristics .